1) Field of the Invention
The field of the invention generally relates to pulp processing and, more specifically, to an improved method and system for treating effluents from cold caustic extraction in connection with a kraft chemical pulping process.
2) Background
Pulp from wood and plant materials has a large number of commercial uses. Although one of the most common uses is in paper manufacturing, pulp can also be used to produce a number of other products including rayon and other synthetic materials, as well as cellulose acetate and cellulose esters, which are used, for example, in the manufacture of filter tow, cloth, packaging films, and explosives.
A number of chemical and mechanical methods exist for processing wood and plant materials in order to manufacture pulp and paper. The basic processing steps include preparing the raw material (e.g., debarking and chipping), separating the wood fibers by mechanical or chemical means (e.g., grinding, refining or cooking) to separate the lignin and extractives from cellulose of the wood fibers, removing coloring agents by bleaching, and forming the resulting processed pulp into paper or other products. In addition to and in connection with pulp and paper manufacturing, paper mills also typically have facilities to produce and reclaim chemical agents, collect and process by-products to produce energy, and remove and treat wastes to minimize environmental impact.
“Pulping” generally refers to the process for achieving fiber separation. Wood and other plant materials comprise cellulose, hemicellulose, lignin and other minor components. Lignin is a network of polymers interspersed between individual fibers, and functions as an intercellular adhesive to cement individual wood fibers together. During the pulping process, lignin macromolecules are fragmented, thereby liberating the individual cellulosic fibers and dissolving impurities that may cause discoloration and future disintegration of the paper or other final product.
The kraft process is a commonly used pulping process. Paper produced from kraft pulping process can be used, for example, to make bleached boxboard and liner board used in the packaging industry. A conventional kraft process treats wood with an aqueous mixture of sodium hydroxide and sodium sulfide, known as “white liquor”. The treatment breaks the linkage between lignin and cellulose, and degrades most of lignin and a portion of hemicellulose macromolecules into fragments that are soluble in strongly basic solutions. This process of liberating lignin from surrounding cellulose is known as delignification. The soluble portion is thereafter separated from the cellulose pulp.
FIG. 1 shows a flow diagram of a conventional kraft process 100. The process 100 involves feeding wood chips (or other organic pulp-containing raw materials) 118 and alkaline solutions into a high-pressure reaction vessel called a digester to effect delignification, in what is referred to as a “cooking” stage 121. The wood chips are combined with white liquors 111, which may be generated from downstream processes or provided from a separate source. Delignification may take several hours and the degree of delignification is expressed as the unitless “H factor”, which is generally defined so that cooking for one hour in 100° C. is equivalent to an H factor of 1. Because of the high temperature, the reaction vessel is often pressurized due to the introduction of steam. Towards the end of the cooking step, the reaction vessel is reduced to atmospheric pressure, thereby releasing steam and volatiles.
The white liquor used in the cooking may be, for example, a caustic solution containing sodium hydroxide (NaOH) and sodium sulfide (Na2S). The property of the white liquor is often expressed in terms of effective alkali (“EA”) and sulfidity. Effective alkali concentration may be calculated as the weight of sodium hydroxide plus one-half the weight of sodium sulfide, and represents the equivalent weight of sodium hydroxide per liter of liquor, expressed in gram per liter. Effective alkali charge as sodium hydroxide represents the equivalent weight of sodium hydroxide per oven-dried weight of wood, expressed in percentage. Sulfidity is the ratio of one-half the weight of sodium sulfide to the sum of the weight of sodium hydroxide and one-half the weight of sodium sulfide, expressed in percentage.
After cooking, a brown solid cellulosic pulp, also known as “brown stock,” is released from the digester used in the cooking stage 121, and is then screened and washed in the washing and screening process 122. Screening separates the pulp from shives (bundles of wood fibers), knots (uncooked chips), dirt and other debris. Materials separated from the pulp are sometimes referred to as the “reject” and the pulp as the “accept.” Multi-stage cascade operations are often utilized to reduce the amount of cellulosic fibers in the reject stream while maintaining high purity in the accept stream. Further fiber recovery may be achieved through a downstream refiner or reprocess of sieves and knots in the digester.
The brown stock may then be subject to several washing stages in series to separate the spent cooking liquors and dissolved materials from the cellulose fibers. The spent cooking liquor 112 from the digester employed in the cooking stage 121 and the liquor 113 collected from the washing and screening process 122 are commonly both referred to as “black liquor” because of their coloration. Black liquor generally contains lignin fragments, carbohydrates from the fragmented hemicelluloses, and inorganics. Black liquor may be used in addition to white liquor in the cooking step, as illustrated for example in FIG. 1 by the arrow representing black liquor 113 produced in the washing and screening process 122 and transferred to the cooking stage 121. Black liquor 135 from an accumulator tank (not shown in FIG. 1) may also be fed to the digester as part of the cooking stage 121, if needed to achieve the appropriate alkaline concentration or for other similar purposes.
The cleaned brown stock pulp 131 from the washing and screening process 122 may then be blended with white liquor 114 and fed into a reaction vessel to further remove dissolved materials such as hemicellulose and low molecular weight cellulose. An exemplary separation method is the so-called cold caustic extraction (“CCE”) method, and is represented by CCE reaction stage 123 in FIG. 1. The temperature at which the extraction is effected may vary but is typically less than 60° C.
The purified pulp 132 from the reactor used in the CCE reaction stage 123 is then separated from spent cold caustic solution and dissolved hemicellulose, and washed several times in a second washing and separation unit in a CCE washing stage 124. The resulting purified brown pulp 133 with relatively high alpha cellulose content, still containing some lignin, continues to a downstream bleaching unit for further delignification. In some pulp production processes, bleaching is performed before the CCE reaction stage 123 and the CCE washing stage 124.
It is desirable in a number of applications, such as the manufacture of synthetic materials or pharmaceutical products, to have pulp of very high purity or quality. Pulp quality can be evaluated by several parameters. For example, the percentage of alpha cellulose content expresses the relative purity of the processed pulp. The degrees of delignification and cellulose degradation are measured by Kappa Number (“KN”) and pulp viscosity respectively. A higher pulp viscosity indicates longer cellulose chain length and lesser degradation. Pulp solubility in 18 wt % sodium hydroxide aqueous solutions (“S18”) provides an estimate on the amount of residual hemicellulose. Pulp solubility in 10 wt % sodium hydroxide aqueous solution (“S10”) provides an indication on the total amounts of soluble matters in basic solutions, which include the sum of hemicellulose and degraded cellulose. Finally, the difference between S10 and S18 determines the amount of degraded cellulose.
In a conventional process, the filtrate 116, also referred to as the CCE alkaline filtrate, from the CCE washing and separation stage 124 comprises both the spent cold caustic solution and the spent washing liquid from the washing and separation stage 124. This filtrate 116 often contains substantial amounts of high molecular hemicellulose. When filtrate with high hemicellulose content is used as part of the cooking liquor in the digester of the cooking stage 121, hemicellulose may precipitate out of the solution and deposit on the cellulosic fibers. This can prevent high quality pulp from being achieved. On the other hand, certain applications—such as high quality yarn or synthetic fabrics, materials for liquid crystal displays, products made with acetate derivatives, viscose products (such as tire cord and special fibers), filter tow segments used in cigarettes, and certain food and pharmaceutical applications—desire pulps containing a minimal amount of redeposited hemicelluloses and alpha cellulose content.
Some portion of the CCE alkaline filtrate 116 may be reused in the cooking stage 121, while the remainder is sent to a recovery area 134 in order to control the risk of hemicelluloses redeposition in the cooking stage 121. In the recovery area 134, the diverted CCE alkaline filtrate 116 may be combined with excess black liquor, concentrated and combusted in a recovery boiler to consume the organics and recover inorganic salts, or else was taken to another pulping line, or a combination of both. A new alkali source may then be needed to replace the CCE filtrate and black liquor sent to the recovery area 134, in order to maintain proper alkali balance in the cooking stage 121. The recovery process and the provision of a new alkali source tends to result in increased production costs.
There exists a need for a pulp processing method and system that results in a dissolving pulp with very high alpha cellulose content. There further exists a need for a pulp processing method and system that provides increased efficiency and permits efficient use of the CCE filtrate while minimizing hemicellulose deposition during cooking.